General aspects of thermoelectric effects in spin valves consisting of two magnetic layers separated by a nonmagnetic spacing layer are considered, with the main focus on the spin Seebeck effects. The Seebeck and spin Seebeck effects are considered in both current-in-plane and current-perpendicular-to-plane geometries. The corresponding thermopower and spin thermopower in the macroscopic limit of electronic transport are also considered. Physical origin of the spin effects is discussed in detail.

A crystal can undergo several phases from higher to lower, and from lower to higher symmetries when temperature or pressure alters. Under su cient high pressure wurtzite ZnO and GaN can be transformed to higher cubic symmetry structure [1]. This kind of transitions are of the rst order. When temperature changes and reaches Curie temperature, the density of state changes continuously and the crystal experiences the second-order phase transition (S.O.Ph.Tr) [2]. The group of the crystal after transition is a subgroup of initial symmetry G0 (active irreducible representations of O h group of rocksalt crystals are Γ1−,Γ2±,Γ3−,Γ4±,Γ5− L1±,2±,3± X1−,2±,3±,4−,5− W1,2,3,4). The Landau-Lifshitz (L L) [3] theory provides possible subgroups after transition and phonon symmetries [(active irreducible representaions (irreps) of G0]. The theory is well established and can be found in many text books [4]. In magnetic crystals the states are classi ed according to irreducible corepresentations (coreps) [5]. Therefore, the phase transitions in these crystals are supposed to be analysed in terms of active coreps of phonons. Consequently the L L theory for non magnetic crystals must be reformulated to magnetic compounds. The L L criteria have been re-written for magnetic compounds by Cracknell [4]. However the modi ed L L criteria has never been applied to any magnetic phase transitions. Generally, the description of transitions in terms of ordinary L L theory was applied to magnetic media. The question is whether such approach by irreps instead of coreps is adequate or not. Here we investigate transitions in magnetic calcium aluminium thosilicate in terms of our modi ed L L theory using corep methods , as well as traditional irrep

Possible symmetry of modes those may cause transitions in magnetic and non magnetic crystals as well as lower space subgroup symmetries of the crystals have been found. The Landau-Lifshitz theory for non magnetic crystals has been reformulated for magnetic crystals and has been applied to Ca3Al2(SiO4)(O-h(10)). Some experimental data confirm our results.

We investigate the second-order phase transitions in non-magnetic wurtzite ZnO and magnetic doped with cobalt. Using reformulated Landau Lifshitz-theory of second-order phase transitions and our computer program, we have found all possible lower space group symmetries of ZnO and ZnO doped with Co as well as symmetries of vibration modes which may cause structural phase transitions. We interpret the Raman phonon modes of magnetic doped ZnO according to corepresentations of the magnetic space group P6(3)'m(c)' (ZnO magnetic). Some experimental techniques like X-ray diffraction, reflectivities and Raman spectroscopies can verify our theoretical results.

At sufficiently high pressures, wurtzite structure zinc oxide (W‐ZnO) can be transformed to the cubic rocksalt (R‐ZnO) structure. The R‐ZnO exhibits semiconductor behavior with an indirect wide band gap of . The maximum valence band is found far away from the center of the Brillouin zone (BZ) at high symmetry point L and line Σ, depending on the pressure. The unusual electronic band structure (EBS) of the R‐ZnO leads to several direct and indirect optical transitions which find applications in ultraviolet optoelectronic devices. We have investigated radiative and non‐radiative symmetry restricted selection rules, as well as inter‐ and intra‐valley scattering processes.

This is one of the first attempts of making WO3 thick film selective at low operating temperature by placing graphene sheets directly on the film surface. The WO3 film was prepared by DC magnetron reactive sputtering of a pure tungsten target and annealing in air. The thickness of the film averaged 507 nm from FIB cross-sectioning and STEM measurement. The film was characterized with XRD, XPS and Raman spectroscopy. Graphene was synthesized on copper foil using plasma enhanced chemical vapor deposition of methane gas, which was found to produce three monolayers of graphene sheets according to Raman spectroscopic analysis. The graphene was transferred onto the WO3 film and annealed. It was found that the pure WO3 sensed low concentrations of nitrogen dioxide at 30°C and 100°C operating temperature. The selectivity towards nitrogen dioxide over ammonia and carbon monoxide was achieved with graphene on the tungsten trioxide film at the compromise of sensitivity at both operating temperatures. The analysis of the graphene/WO3 composite orientation may lead to improved sensor capabilities as well as better understanding of the sensing mechanism.

The possibility of obtaining vanadium dioxide (VO2) [wherein the vanadium ionic state is 4(+)] from a precursor of ammonium metavanadate (NH4VO3) bearing the ion V5+ is investigated. The reduction is carried out by calcining the NH4VO3 powders in similar concentrations of H-2 flow at varying temperatures. The resulting powders have been studied by several techniques including XRD, Raman spectroscopy, FTIR, TEM, BET and DSC. It is found that remnants of bright yellow V5+ still exist up to calcination temperatures of 100 degrees C after which the sky-blue VO2 dominates at calcination temperatures of 150 degrees C -250 degrees C. There is a population surge of metastable dark-blue V6O13 (where V is in between V4+ and V5+ ionic states) between 250 degrees C and 300 degrees C. However above 350 degrees C the material reverts to the stable V5+ in the yellow orange V2O5. XPS/EDS and VSM confirm the order of appearance to be VO2(150 degrees C) -> V6O13(200 degrees C) -> V2O5 (350 degrees C). (C) 2014 Elsevier B.V. All rights reserved.

Laser pyrolysis was chosen to synthesize tungsten trioxide starting with tungsten ethoxide precursor. The film was found to have a thickness that varied from 205 nm to 1 mu m. X-ray diffraction and Raman spectroscopy confirmed the presence of a mixture of hexagonal and tetragonal phase WO3 in the synthesized film, as well as tungsten bronzes. It was evident that annealing greatly influenced the phases and types of structures formed, and EDXS was carried out in an attempt to quantify the tungstate bronzes to tungsten oxide. I-V curves of the films showed n-type semiconducting behaviour, but the mixed phase appeared to cause a similar behaviour of dopants in a semiconductor. The refractive index decreased with increasing wavelength and gave values of up to 21 at low wavelengths. The average optical band gap was found to be 3.6 eV from UV/Vis spectroscopy. Scanning Electron Microscopy (SEM) showed a mixture of nano-and microstructures after annealing. Nanorod structures were isolated and Pt-contacted using FIB for possible applications such as an active sensing medium in gas sensors. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

WO3 film sputtered on Al2O3 was used to sense ppm concentrations of NO2 and NH3 gas. The optimum working temperature of the amorphous film was found to be 200°C for both the gases, in accordance with literature. However, thermo-gravimetric analysis of the films predicts a different mechanism from that in literature which speculates that there is loss of water molecules from WO3 at 200°C and therefore better sensing response. Thermo-gravimetric analysis shows that there is an increase in weight percentage in air at 200°C, which we speculate to be due to optimum surface oxygen content which leads to better sensing response. Raman spectroscopy at 200°C supports our speculation by showing no structural change in the WO3 compound and no shifting of the ~700 cm1 peak which is indicative of water loss. Furthermore, there was a marked change in the heating-cooling hysteresis at 200°C, which could result from the optimum surface oxygen content changing the electron transport properties The 200°C may also be regarded as a new transition temperature in WO3 although the transition is not structural (electron-phonon coupling) but is electronic in nature (electron-electron correlation) and this transition temperature could be linked to the optimum sensing temperature of WO3.

The selection rules (SR's) for the Kronecker product (KP) of Si and Ge irreducible representations (irreps) are required to determine the intervalley scattering processes. The SR's for transitions between the lowest conduction band minima at Gamma, X and L high symmetry points and the highest maximum of the valence band (VB) in the Brillouin zone of O-h(7) space group symmetry are determined. The symmetry of phonons due to electron-phonon (El-Ph) interaction follows from the KP's: L-1 circle times Gamma(12) (Gamma(2-)), Delta(1) circle times Gamma(12) (Gamma(2-)), L-1(L1+) circle times Delta(1) for Ge and L-1 circle times Gamma(15)(Gamma(4)), Delta(1) circle times Gamma(15) (Gamma(4)), L-1 (L1+) circle times Delta(1) Si.The elements of El-Ph scattering tensors are linear combinations of the Clebsch-Gordon coefficients (CGC's). Here we have computed the coupling coefficients relevant to scattering tensors. Our theoretical results confirm the available experimental data. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The analysis of Raman and Brillouin scattering spectra requires the knowledge of scattering tensors. Based on Birman's method, we compute the Clebsch-Gordan coefficients for first-order Raman and Brillouin scattering for Bi2Se3 with D-3d(5) space group symmetry. The linear combination of Clebsch-Gordan coefficients gives the matrix elements of scattering tensors. Our calculations are useful for interpretation of spectra in Raman and Brillouin scattering measurements.

The matrix elements of the Raman scattering tensor s are constructed from the ClebschGordan coefficients (CGC’s). Using group theoretica l Birman’s method, we calculate the CGC’s of the tetragonal phase tungsten trioxide α-WO3 with the space group 7 4h D . These obtained scattering tensors are used in the interpretation of the Raman spectra .

Scattering matrix for two phonon processes at k = 0 in Si and Ge of O-h(7) symmetry is given. Also diagonalization of spin-orbit interaction Hamiltonian has been computed by means of Clebsh-Gordan coefficients. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Selection rules for inter‐ and intra‐valley scattering processes between the minima of the lowest conduction band Γ, L, M, and A are investigated in wurtzite ZnO. The knowledge of the selection rules is required for the determination of the Clebsch–Gordon coefficients (CGCs). These coefficients diagonalize the Kronecker products (KPs) of relevant irreducible representations of quasi particles participating in scattering processes. The elements of the scattering tensors are the linear combinations of the CGCs. Within this work, the CGCs for ZnO belonging to the space group $C_{6\nu }^4 $ are determined.

A few examples of four-spin Heisenberg systems with dominant antiferromagnetic couplings are considered. All systems can be described by the so-called sublattice Hamiltonian H=SA·SB or its modifications, where SX is the total spin of a sublattice X=A, B. In such a case (eigen)energies are simple functions of the total spin number S, total spins of sublattices SA, SB, and the Hamiltonian parameters (ratios of exchange integrals). Moreover, eigenstates are strictly determined by a coupling scheme assumed and the appropriate Clebsch–Gordan coefficients. In this sense the systems considered are classical ones.

The correlation between ionized donor bound exciton recombinations and neutral donor bound exciton recombinations in ZnO has been investigated. The experimental data obtained by means of magneto-photoluminescence (MPL) concerning charge state and localization energies of ionized and neutral donor bound excitons are in good agreement with theoretical predictions. The optical selection rules in absence and presence of time reversal symmetry (TRS) are investigated. It is shown that the inclusion of extra degeneracy due to TRS reveals a number of new states of the same symmetries and essentially does not change the existing optical selection rules.

We have examined the effect of Time Reversal Symmetry (TRS) on vibrational modes and on the electronic band structure of Si and Ge. Most of the primary non-interacting modes are not affected by TRS. Only phonons originating from high symmetry lines S and A of the Brillouin Zone (BZ) indicate extra degeneracy. Selection rules for some two and three phonons originating from high symmetry lines are determined. The states of electrons and holes described by electronic band structure due to spin-inclusion are assigned by spinor representations of the double space group. Inclusion of the TRS into the band structure results in extra degeneracy of electrons and holes, and therefore optical selection rules suppose to be modified.

Spin torque and current-induced magnetic switching (dynamics) in spin valve nanopillars is considered theoretically in the diffusive transport regime. Basic characteristics of the precessional states are discussed. Current-induced switching in other spin valves is analyzed, particularly in ferromagnetic single-electron transistors based on magnetic molecules.

The Space Symmetry and the Time Reversal Symmetry of vibrational modes in Si, Ge and diamond are investigated. Using Space Symmetry we have derived the Lattice Mode Representation. Reducing it onto phonon species we obtain the symmetry allowed phonons and their degeneracies. Using reality test for irreducible representations according to those the phonons are classified we determine which modes are Time Reversal affected. Comparison with experimental data obtained by neutron scattering is made. The effect of Time Reversal Symmetry on electrons in the conduction band and holes in the valence band as well as on excitons is briefly discussed.

Vibrational states in a crystal are classified according to the irreducible representations (irreps) of the corresponding factor group G(0)(k)/T. The wave vector k runs over the entire Brillouin zone (BZ). For trigonal BZs, the factor groups are determined by the symmetry points F, F, L, T, and the symmetry lines Lambda, Sigma, Y. When the irreps are complex, the time reversal symmetry has to be taken into account. Using the Frobenuis-Schur criterion adapted to space groups with real and complex irreps, we have investigated high symmetry points and lines of the phonons in trigonal crystals: Cr2O3, Fe2O3, Ti2O3, V2O3, FeCO3, CaCO3, CdCO3, MGCO(3), MnCO3, NaCO3 and ZnCO3, with the common space group D-3d(6) (R (3) over barc). We have found several phonons which are influenced by the time reversal symmetry. Therefore, an extra degeneracy of phonons arises. The theoretical results are also compared with available experimental data. (c) 2007 Published by Elsevier Ltd.